This application pertains to the art of power supplies and more particularly to maintaining regulated output power at high ambient temperatures by employing thermal current limiting and back-up power supply techniques in the communication industry.
The invention is particularly applicable to power supply systems which include a backup battery system for supplying power to a load when the primary power supply is operating at a value insufficient to fully supply load requirements. It will be appreciated, however, that the invention has broader applications and may be advantageously employed in other situations that impose similar constraints in the power supply arts.
A variety of regulated power supplies are used in a wide range of environments. Among these are regulated constant voltage output power supplies. In such environments the regulated constant voltage output power supply is provided with overload protection to limit the maximum current which may be drawn from the power supply. Without such protection load current would become excessive resulting in damage to the power supply or to the load. FIG. 1A illustrates a simple block diagram of power supply system A, consisting of a regulated constant voltage power supply 10 used to supply a load 12, where power supply 10 includes current limiter 14.
Two well known types of current limiting are constant current limiting, and foldback current limiting. The first type, constant current limiting, limits the output current to a constant value if the load current attempts to exceed a defined maximum. A graph provided in FIG. 1B illustrates operational characteristics of a constant voltage current limited power supply. When load current 16 reaches its maximum overload rating (e.g. 110% of rated maximum) the power supply 10 switches from a constant voltage output to a constant current output. At this point load current 16 is at its maximum, and voltage 18, which has previously been at a constant 52 volts, begins to decrease 18'.
A more detailed example of the operation of a constant voltage power supply is depicted in FIG. 1C illustrating typical voltage/current characteristics with linear (resistive) load lines 20-26. From this diagram it can be seen that as the load current I.sub.load increases from a low value, where output current I.sub.load intersects load line 20 (high resistance), to its maximum normal load current I.sub.max, where output current I.sub.load intersects load line 24 (medium resistance), load current I.sub.load increases at a constant voltage along characteristic points 20', 22', 24'. At these points all currents and voltages are within the normal working range of the power supply.
When a limiting current value is reached, i.e. point 24', further increase in the load current I.sub.load stops. Hence, as load resistance continues to fall toward zero, the load current remains constant and the voltage falls toward zero (see characteristic point 26').
As illustrated by FIG. 1D, foldback current limiting, is similar to constant current limiting, except that as the voltage is reduced, as a result of the load resistance moving towards zero, load current I.sub.load is also induced to fall. The resistive load line 30 has its point of origin at zero and load current I.sub.load is proportional to the voltage. As the resistive load line 30 shifts, to 30', the straight load line 30 which starts vertically at zero load (i.e., infinite resistance) swings clockwise around the origin to a horizontal position, representing a short circuit, i.e. a zero resistance situation. Thus, straight resistive load line 30 crosses the foldback characteristic of the power supply at only one point. In FIG. 1D, as the load current I.sub.load increases from zero, the voltage initially remains constant at the stabilized 52-Volt output. However, when the maximum limiting current I.sub.max is reached at 32', any further attempt to increase the load (i.e. reduction of load resistance) results in a reduction in both output voltage and load current I.sub.load so operating points, 34, 36 are maintained within a working range which will not cause damage to the power supply or load. Under short circuit conditions, only a small current I.sub.sc flows in the output terminals.
In the above purely resistive loads, as there is only one operating point, control of the system is somewhat simplified. However, when the load is a constant power load such as 12 of FIG. 1A, a substantially different type of load line 38, depicted in FIG. 1E exists. In a constant power load system such as that of the present invention, there are two operating points 40 and 42. While systems having such dual operating points are useful when operating within a normal working range, when operating at extreme conditions load line 38 can, as shown by load line 38', be moved out of the normal operating range, i.e. to points 40', 42'. In such a situation, instability of an entire system can occur. Therefore, in constant power load environments as that of the present invention, maintaining the system within the normal working range of power supply 10 is an important consideration.
While the above described current limiting techniques are useful in protecting power supplies from damage, they may also result in a power supply being forced to enter an "offstate" where no output is generated. In this situation a connected load does not receive the power it requires and in turn shuts down or operates in an unstable manner.
As illustrated in FIG. 2, power supply system B is provided with back-up battery supply system 44, including back-up batteries 46. This system is similar to system A of FIG. 1A except that battery back-up system 44 has been operatively connected between the power supply 10 and constant power load 12. In known battery back-up systems, the back-up battery is selectively switched to replace the output from the regulated power supply upon failure of an AC source 48 or upon some other event that undesirably effects the power supply output. Back-up battery system 44 is continuously connected, or "floated", directly across the output of regulated power supply 10. Through this arrangement, when due to high ambient temperature or other event the power supply enters an off-state, back-up battery power system 44, including back-up battery 46, is used to supply constant power load 12. It is to be appreciated back-up battery 46 may consist of a single battery or of a plurality of batteries.
A system which provides power to constant power loads and which has current limiting does, however, have drawbacks. In such a system, when a predetermined temperature is sensed, a thermal protection system causes the regulated voltage power supply to immediately shut down. Particularly, when the predetermined temperature is reached the system immediately enters an off-state until the heat which caused the increase in temperature has dissipated. Thereafter, an automatic turn-on circuit restarts the constant voltage power supply. In systems having such thermal protection, the back-up battery system will supply power to the load when the power supply has been taken off-line. Battery back-up systems, however, have a finite supply capability, therefore, in environments where high ambient temperatures repeatedly cause the thermal protection system to place the regulated power supply into the off-state or, if other factors cause the power supply to frequently enter the off-state, the back-up battery can become discharged to such a level that it will not be able to fully supply load requirements.
It has been deemed desirable to develop a regulated power supply system with battery back-up and thermal current limiting, wherein when a predetermined ambient temperature is reached output from the regulated power supply decreases but does not enter an off state; and to develop a system where the output of the regulated power supply is supplemented by the battery back-up system when the power supply is in a reduced operation state.